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Understanding Mechanisms of Asphaltene Adsorption from Organic Solvent on Mica Anand Natarajan, Natalie Kuznicki, David Harbottle, Jacob H Masliyah, Hongbo Zeng, and Zhenghe Xu Langmuir, Just Accepted Manuscript • Publication Date (Web): 30 Jun 2014 Downloaded from http://pubs.acs.org on June 30, 2014
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Understanding Mechanisms of Asphaltene Adsorption from Organic Solvent on Mica Anand Natarajan, Natalie Kuznicki, David Harbottle, Jacob Masliyah, ∗
Hongbo Zeng , Zhenghe Xu
∗
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2V4, Canada. *E-mail:
[email protected]; phone: 780-492-1044; fax: 780-492-2881; and e-mail:
[email protected]; phone: 780-492-7667; fax: 780-492-2881.
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ABSTRACT
The adsorption process of asphaltene onto molecularly smooth mica surfaces from toluene solutions of various concentrations (0.01 - 1 wt%) was studied using a Surface Forces Apparatus (SFA). Adsorption of asphaltenes onto mica was found to be highly dependent on adsorption time and concentration of the solution. The adsorption of asphaltenes led to an attractive bridging force between the mica surfaces. The adsorption process was identified to be controlled by diffusion of asphaltenes from the bulk solution to the mica surface with a diffusion coefficient on the order of 10-10 m2/s at room temperature, depending on asphaltene bulk concentration. This diffusion coefficient corresponds to a hydrodynamic molecular radius of approximately 0.5 nm, indicating that asphaltene diffuses to mica surfaces as individual molecules at very low concentration (e.g. 0.01 wt%). Atomic force microscopy (AFM) images of the adsorbed asphaltenes on mica support the results of the SFA force measurements. The results from the SFA force measurements provide an insight on the molecular interactions (e.g. steric interaction, bridging attraction as a function of distance) of asphaltenes in organic media and hence their roles in crude oil and bitumen production. Keywords: asphaltenes, adsorption, interaction forces in solvent, Surface Forces Apparatus
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INTRODUCTION Interfacial phenomena govern the properties of solids, liquids and gases in pharmaceuticals, food products, cosmetics, and petroleum processing, to name a few. From interfacial studies, one can determine the fundamental mechanisms and kinetics of film formation, surfactant adsorption, and film rupture that ultimately govern the behaviour of dispersion systems. Of extreme importance in the petroleum industry is the formation of stable water-in-oil (w/o) emulsions that result from the adsorption of bituminous compounds at an oil-water interface. The interactions and assembly of adsorbed molecules or their aggregates lead to formation of a protective layer that resists droplet-droplet coalescence.1–11 Stable emulsions have been a major challenge in petroleum production, affecting process throughput and downstream operations such as upgrading. It is necessary to develop knowledge on the formation of these protective layers to design suitable protocols for film disruption and prevention of stable emulsion formation. Polyaromatic asphaltenes containing a significant fraction of heteroatoms such as oxygen, nitrogen and sulphur are commonly considered the problematic component of crude oil and bitumen. They readily partition at the liquid-liquid and solid-liquid interfaces. The complex nature of asphaltene molecules makes it difficult to define their molecular structures. As a result asphaltenes are poorly defined as a solubility class material that is soluble in toluene but insoluble in alkanes such as n-heptane.12,13 Due to the presence of heteroatoms, asphaltenes are partially polar, and have a tendency to adsorb at the oil-water and oil-solid interface. The adsorption of asphaltenes on solids such as pipelines and clay particles create added processing difficulties. While pipeline deposition can lead to pipe plugging, adsorption onto clay solids modifies their wettability, decreasing efficiency of oil liberation from the solids and increasing solids partitioning at the oil-water interface.14-20 In oil production, the formation of a
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complex multiphase dispersion layer in the middle of oil-water separation vessel, known as rag layers, is frequently encountered.21,22 The rag layers are extremely stable multiemulsions with the stabilizing species identified as surfactants, asphaltenes and inorganic fine particles. The inorganic fine particles are predominantly iron-containing solids that readily associate with organic compounds such as asphaltenes to form biwettable solids which preferentially stabilize w/o emulsions. The presence of this highly viscous rag layer with medium density prevents the separation of emulsified water from oil or vice versa. The adsorption of asphaltenes on mineral surfaces has commonly been studied by quantitatively determining the amount of asphaltene adsorbed using i) solution depletion method,23,24 ii) quartz crystal microbalance,25,26 iii) photothermal surface deformation spectroscopy27 and iv) thermal gravimetric method, or qualitatively by contact angle measurements.16,17 Common clay minerals used in adsorption studies are kaolinite, illite, montmorillonite, mica and chlorite. The common non-clay minerals used in adsorption studies are quartz, calcite, feldspar, limestone, and dolomoite. Dudasova et al.28 studied the adsorption of asphaltenes from five different petroleum sources onto different organic surfaces and observed Langmuir-type adsorption. They concluded that the adsorption depends more upon the solid surface than upon the origin of the asphaltenes and that the heteroatomic species in asphaltenes were important for surface adsorption. According to Buckley,29 the driving force for asphaltene adsorption on inorganic solids in the absence of water is mainly through polar interactions between oil and a solid, which leads to surface precipitation of asphaltenes. Mica, because it can be cleaved to provide uniform and molecularly smooth surfaces has been used to study asphaltene adsorption.14,30 Recently, Adams published a comprehensive review on the chemical and physical nature of asphaltenes and
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corresponding characteristics of asphaltene adsorption.37 The above research provided valuable information on stabilization mechanisms and adsorption kinetics of asphaltenes. Due to the complexity of asphaltene molecules and limitations of some of the techniques described, probing molecular mechanisms of asphaltene adsorption on inorganic solid surfaces from organic solvents and subsequent association remains rather limited. In all of these studies, asphaltenes are extracted from crude oils and solubilised in mixtures of toluene and n-alkanes. According to Buckley,28 the driving force for asphaltene adsorption on inorganic solids in the absence of water is mainly through polar interactions between oil and a solid, which leads to surface precipitation of asphaltenes. Determining the organization of asphaltenes at a solid-liquid interface has been the subject of numerous studies generally involving determination of adsorption isotherms. Several authors have reported Langmuir-type adsorption isotherms, indicating the formation of a monolayer of asphaltenes at the toluene-quartz interface.17,29 Whereas, others have found stepwise or linear increase in adsorption, suggesting a multilayer adsorption process, for example, toluene to glass, and toluene to quartz/dolomite/kaolin/calcite.27,30,31 Ellipsometry studies confirmed the presence of multilayer asphaltenes adsorbed on solids.32 Acevedo et al.27,33 suggested that a weakly aromatic asphaltene that is stable in its crude oil would form a single layer whereas a strongly aromatic asphaltene, which exhibits a tendency to precipitate, would form a multilayer. The kinetic aspect of asphaltene adsorption on metal surfaces has also been studied. With quartz crystal microbalance measurements, Xie and Karan34 showed that the initial asphaltene adsorption process on gold surface is controlled by the diffusion of asphaltenes from the bulk solution to the substrate surface. The equilibrium state is then reached only after an extended
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adsorption period, which can be as long as several days, as also observed by Acevedo et al.33 Using solution depletion method, Goual et al.26 determined two distinct regimes of asphaltene adsorption on silica surface as a function of asphaltene concentration: at low bitumen concentration, the system did not reach a steady-state of adsorption because the adsorbed amount of asphaltenes was still increasing linearly even after 8 h of adsorption; whereas at high bitumen concentration a steady-state of adsorption was reached after just few min (
